Iodine Anions beyond −1: Formation of Li_<i>n</i>I (<i>n</i> = 2–5) and Its Interaction with Quasiatoms

Novel phases of Li_<i>n</i>I (<i>n</i> = 2, 3, 4, 5) compounds are predicted to form under high pressure using first-principles density functional theory and an unbiased crystal structure search algorithm. All of the phases identified are thermodynamically stable with respect to decomposition into elemental Li and the binary LiI at a relatively low pressure (≈20 GPa). Increasing the pressure to 100 GPa yields the formation of a high pressure electride where electrons occupy interstitial quasiatom (ISQ) orbitals. Under these extreme pressures, the calculated charge on iodine suggests the oxidation state goes beyond the conventional and expected −1 charge for the halogens. This strange oxidative behavior stems from an electron transfer going from the ISQ to I^– and Li⁺ ions as high pressure collapses the void space. The resulting interplay between chemical bonding and the quantum chemical nature of enclosed interstitial space allows this first report of a halogen anion beyond a −1 oxidation state

Nitrophosphorene: A 2D Semiconductor with Both Large Direct Gap and Superior Mobility

A new two-dimensional phosphorus nitride monolayer (<i>P</i>2₁/<i>c</i>-PN) with distinct structural and electronic properties is predicted based on first-principle calculations. Unlike pristine single-atom group V monolayers such as nitrogene, phosphorene, arsenene, and antimonene, <i>P</i>2₁/<i>c</i>-PN has an intrinsic direct band gap of 2.77 eV that is very robust against the strains. Strikingly, <i>P</i>2₁/<i>c</i>-PN shows excellent anisotropic carrier mobility up to 290 829.81 cm² V^–1 s^–1 along the <i>a</i> direction, which is about 18 times that in monolayer black phosphorus. This put <i>P</i>2₁/<i>c</i>-PN way above the general relation that carrier mobility is inversely proportional to bandgap, making it a very unique two-dimensional material for nanoelectronics devices

Unexpected Trend in Stability of Xe–F Compounds under Pressure Driven by Xe–Xe Covalent Bonds

Xenon difluoride is the first and the most stable of hundreds of noble-gas (Ng) compounds. These compounds reveal the rich chemistry of Ng’s. No stable compound that contains a Ng–Ng bond has been reported previously. Recent experiments have shown intriguing behaviors of this exemplar compound under high pressure, including increased coordination numbers and an insulator-to-metal transition. None of the behaviors can be explained by electronic-structure calculations with fixed stoichiometry. We therefore conducted a structure search of xenon–fluorine compounds with various stoichiometries and studied their stabilities under pressure using first-principles calculations. Our results revealed, unexpectedly, that pressure stabilizes xenon–fluorine compounds selectively, including xenon tetrafluoride, xenon hexafluoride, and the xenon-rich compound Xe₂F. Xenon difluoride becomes unstable above 81 GPa and yields metallic products. These compounds contain xenon–xenon covalent bonds and may form intercalated graphitic xenon lattices, which stabilize xenon-rich compounds and promote the decomposition of xenon difluoride

Predicted Lithium–Boron Compounds under High Pressure

High pressure can fundamentally alter the bonding patterns of light elements and their compounds, leading to the unexpected formation of materials with unusual chemical and physical properties. Using an unbiased structure search method based on particle-swarm optimization algorithms in combination with density functional theory calculations, we investigate the phase stabilities and structural changes of various Li–B systems on the Li-rich regime under high pressures. We identify the formation of four stoichiometric lithium borides (Li₃B₂, Li₂B, Li₄B, and Li₆B) having unforeseen structural features that might be experimentally synthesizable over a wide range of pressures. Strikingly, it is found that the B–B bonding patterns of these lithium borides evolve from graphite-like sheets in turn to zigzag chains, dimers, and eventually isolated B ions with increasing Li content. These intriguing B–B bonding features are chemically rationalized by the elevated B anionic charges as a result of Li→B charge transfer

On the Stereochemical Inertness of the Auride Lone Pair: Ab Initio Studies of AAu (A = K, Rb, Cs)

The “lone” 6s electron pair often plays a key role in determining the structure and physical properties of compounds containing sixth-row elements in their lower oxidation states: Tl⁺, Pb²⁺, and Bi³⁺ with the [Xe]­4f¹⁴5d¹⁰6s² electronic configuration. The lone pairs on these ions are associated with reduced structural symmetries, including ferroelectric instabilities and other important phenomena. Here we consider the isoelectronic auride Au^– ion with the [Xe]­4f¹⁴5d¹⁰6s² electronic configuration. Ab initio density functional theory methods are employed to probe the effect of the 6s lone pair in alkali-metal aurides (KAu, RbAu, and CsAu) with the CsCl structure. The dielectric constants, Born effective charges, and structural instabilities suggest that the 6s lone pair on the Au^– anion is stereochemically inert to minor mechanical and electrical perturbation. Pressures greater than 14 GPa, however, lead to reorganization of the electronic structure of CsAu and activate lone-pair involvement and Au–Au interactions in bonding, resulting in a transformation from the cubic CsCl structure type to an orthorhombic <i>Cmcm</i> structure featuring zigzag Au–Au chains

<i>N</i>‑Alkyldinaphthocarbazoles, Azaheptacenes, for Solution-Processed Organic Field-Effect Transistors

Substituted <i>N</i>-alkyldinaphthocarbazoles were synthesized using a key double Diels–Alder reaction. The angular nature of the dinaphthocarbazole system allows for increased stability of the conjugated system relative to linear analogues. The <i>N</i>-alkyldinaphthocarbazoles were characterized by UV–vis absorption and fluorescence spectroscopy as well as cyclic voltammetry. X-ray structure analysis based on synchrotron X-ray powder diffraction revealed that the <i>N</i>-dodecyl-substituted compound was oriented in an intimate herringbone packing motif, which allowed for p-type mobilities of 0.055 cm² V^–1 s^–1 from solution-processed organic field-effect transistors

<i>N</i>‑Alkyldinaphthocarbazoles, Azaheptacenes, for Solution-Processed Organic Field-Effect Transistors

Substituted <i>N</i>-alkyldinaphthocarbazoles were synthesized using a key double Diels–Alder reaction. The angular nature of the dinaphthocarbazole system allows for increased stability of the conjugated system relative to linear analogues. The <i>N</i>-alkyldinaphthocarbazoles were characterized by UV–vis absorption and fluorescence spectroscopy as well as cyclic voltammetry. X-ray structure analysis based on synchrotron X-ray powder diffraction revealed that the <i>N</i>-dodecyl-substituted compound was oriented in an intimate herringbone packing motif, which allowed for p-type mobilities of 0.055 cm² V^–1 s^–1 from solution-processed organic field-effect transistors